The present invention relates to a substrate processing method and apparatus for mainly mass-producing SOI substrates having uniform quality and, more specifically, forming porous silicon used to manufacture SOI substrates, and an SOI substrate.
Porous silicon was found by A. Uhlir and D. R. Turner who were studying electropolishing of single-crystal silicon biased to a positive potential in an aqueous solution of hydrofluoric acid (to be abbreviated as "fluoric acid" hereinafter).
Later, to exploit the excellent reactivity of porous silicon, application to element isolation in which a thick insulating structure need be formed in manufacturing a silicon integrated circuit was examined, and a full isolation technology using a porous silicon oxide film (FIPOS: Full Isolation by Porous Oxidized Silicon) was developed (K. Imai, Solid State Electron 24, 159, 1981). This is the first example of application of porous silicon to the SOI (Silicon On Insulator) technology.
Recently, an applied technology to direct bonding in which a silicon epitaxial layer grown on a porous silicon substrate is bonded to an amorphous substrate or single-crystal silicon substrate via an oxide film has been developed (Japanese Patent Laid-Open No. 5-21338).
Details of this technology will be described below. First, a first substrate is etched in an electrolytic solution represented by HF solution to form a porous silicon layer on the surface of the substrate. A single-crystal silicon film is epitaxially grown on the porous silicon layer. This layer is grown as a non-porous single-crystal thin film although the underlying layer is porous. Subsequently, the surface of the epitaxial layer is oxidized. After cleaning, the oxide film surface and a second substrate are bonded and integrated by a heat treatment. The resultant structure is ground from the lower surface side of the first substrate to expose the porous layer. Finally, the porous silicon layer exposed to the surface is removed by etching while leaving the single-crystal silicon film, thereby obtaining a substrate having an SOI structure.
Conventionally, however, when the porous silicon layer is removed by etching, the porous silicon layer partially remains on the single-crystal silicon film.